Trunked communication systems today afford an efficient use of the limited frequency spectrum. Organizations, both private and public, have utilized and tailored trunked systems for their individual communication needs.
In known trunked communication system, a central controller allocates a limited number of communication channels among many radios. Generally, within the trunked system, the many users of the radios are organized into talk-groups as well as individual units. Accordingly, each radio has an individual identification (ID) code and one or more group IDs. Since the number of communications channels is far less than the number of radios, the radios must request and be granted a channel before they may communicate. To do this, the central controller monitors a control channel for channel request from the several radios. Upon receipt of a request, the controller assigns an available channel (if any) to handle the conversation.
Within a particular area, the Federal Communications Commission (F.C.C.) grants a limited number of licenses for high power radio transmission. Such users are commonly referred to as "primary user", that communicate over "primary channels". In policing the use of these primary channels, the F.C.C. enforces stringent administrative requirements, including the transmit frequency, the transmitted power level, spectral purity of the transmitted signal, as well as a myriad of administrative requirements.
In addition to high power radio licensing, over 200 frequencies are allocated by the F.C.C. as low power industrial (LPI) frequencies. These frequencies reside in the 450 megahertz (MHz) to 470 MHz band, and thus, occupy the same band as conventional land mobile frequencies and the UHF business band. To qualify as an LPI radio, transmitter power is limited to two (2) watts. Comparatively, primary channel radios typically have a transmitter power of 50 Watts, although some portable primary radios may have transmitter powers as low as 1 Watt.
Each LPI channel has a 25 KHz bandwidth allocation, but are offset from the primary (high power) channels so as to minimize interference. In this regard, LPI users are regarded as secondary users. That is, an LPI system can use a selected frequency within the LPI band so long as primary users are not interfered with. If a transmitted LPI signal interferes with a primary channel, the LPI user must abandon the channel. Conversely, if a primary user interferes with an LPI channel, the LPI user can remain on the channel if they are willing to tolerate the primary user's interference. Otherwise, the LPI user can abandon the channel in favor of another LPI channel.
To partially compensate for these inconveniences, the FCC has relaxed the requirements for obtaining LPI licenses. Current FCC rules allow an LPI channel user to license several channels simultaneously without the stringent administrative requirements imposed upon high power radios. In fact, licenses for more LPI channels than a contemporary communication system can accommodate are relatively easy to obtain. Thus, a need exists for an LPI communication system that can manage and control several LPI communication channels within the rules and regulations established by the regulatory authorities.